The Colorado Geological Survey maps where landslides occur in Colorado. This is their Colorado Landslide Inventory map.

The area where the Collbran mudslide happened has seen similar slides in the past. Geologists say relatively weak rock and steep terrain create a recipe for such natural disasters. Still, Colorado in general is less vulnerable to slides than wetter areas, like the west coast. Aspen Public Radio's Marci Krivonen reports.

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The Colorado Geological Survey began mapping landslides near Collbran in the 1980’s. They discovered the area where this debris flow happened was prone to slides.

"What we found is that there are very old landslides as well as some recent landslides and debris flows. So, it’s an area where you would expect that to continue and we’re not real surprised that it happened in this area," says Karen Berry.

She's the Director of the Colorado Geological Survey. Landslides are more common on Colorado’s Western Slope, but they can happen anywhere in the state, even on the Eastern Plains. Berry says most slides in Colorado are smaller than the one near Collbran and, often happen in remote and undeveloped areas.

"In a lot of places we have much smaller landslides - they aren’t as wide, they don’t flow as long, and they aren’t as deep-seated as this one, so we do have smaller landslides all over the state."

The Collbran slide area is full of soft sandstone and shale rock that are relatively weak. The geologic formations at the slide are found throughout large parts of Garfield and Mesa counties. Edwin Harp is a research geologist with the USGS. He says the rock in these formations erodes easily.

"This is the kind of shale that you could probably dig with a shovel or possibly an excavator, things like that."

Colorado is generally susceptible to landslides but, they are more likely in wetter states like California and Washington. Harp says any state with steep slopes will see landslides and a number of things could set them off.

"Landslides are triggered by rainfall, snowmelt, earthquakes. And, in some cases, landslides occur and we don’t really know what triggered them," he says.

The USGS sent two geologists to Collbran to evaluate the slide and get to the bottom of what caused it. The Colorado Geological Survey has found landslides in the past in this area usually occur in spring to early summer, when runoff and groundwater levels are high.